High speed audio/video communication systems involve point-point links for high-speed data transfer from a data source such as a camera or a video player and a display. These systems require dedicated point-point links to transfer video data while requiring one or more separate physical links to transfer control information back to the source. The configuration data is usually sent to provide control information for the cameras like zoom, tilt, etc.
One area in which such systems are seeing increasing use is that of automotive media communications for the front seat or dashboard (such as rear/side cameras) and rear seat (such as a DVD player). What is needed is both display data in one direction, and bi-directional control (e.g., camera tilt/zoom and DVD control player). One conventional communication link uses five wire pairs (three for data, one for a clock and one for control). Another conventional communication link uses two wire pairs (one for data with an embedded clock and one for control).
An exemplary application is a communication link between a rear view camera and dashboard display. The camera communicates rear view video data to the dashboard display over the forward channel. Control commands, e.g., pan, tilt or zoom, are communicated back to the camera over the back channel by being modulated onto the forward channel. The camera responds with an acknowledgement signal (ACK) via the forward channel control frame.
In the context of automotive infotainment system, a control channel is primarily used in two cases: safety and driver assist systems, and rear seat entertainment systems.
In safety and driver assist systems, a forward channel carries data from a camera to a head unit for further image processing. A control channel carries management data back from the head unit to camera. Typical control commands include commands to control the camera, e.g., pan, tilt, zoom, etc. A conventional system like this uses two pairs of wires, one for carrying high-speed forward channel data and the other to carry control information.
In rear seat entertainment systems, the forward channel carries high-speed data such as high definition (HD) video from a DVD or Blu-Ray player. The control channel is used to exchange keys for content protection and display aspect ratio information with the video source for automatic video formatting.
Typical figures of merit for evaluating the efficiency of a control channel are latency, electromagnetic interference (EMI) and number of cables and connectors. Regarding latency, in driver assist applications where detecting any obstructions on the road in the front or to the rear of a car or when developing lane-departure warning systems, control channel latency is a very important factor as an evasive action is required to be taken in time to prevent any accidents or injuries. Regarding EMI, automotive systems have stringent emissions specifications. High full-swing TTL/CMOS levels that go around wires inside an automobile can create strong EMI issues that can interfere with other electronic systems that control engine operation. Regarding the number of cables and connectors, a large number of cables and connectors add weight and costs.
Three types a control channel architectures have been used: separate wires, blanking transmission and common-mode modulation. Regarding separate wires, control channel information is transferred via a separate wire or cable. While this offers very good latency, digital switching transients add to EMI and the need for extra cable and connectors adds weight and costs.
Referring to FIG. 1, a conventional embodiment of such a bi-directional communication link 10 includes a forward channel driver 12, a back channel receiver 14, a forward channel receiver 32 and a back channel driver 34, all interconnected substantially as shown. In accordance with well known techniques, the differential forward channel signal 11f is amplified by the forward channel driver 12 to provide the differential forward channel signal 13 across a termination resistance 16 for coupling via coupling capacitors 20p, 20n to the differential signal line 30. At the other end, the signal is further coupled via coupling capacitances 40p, 40n to a termination resistance 36, following which the forward channel signal is amplified by the forward channel receiver 32 to provide the forward channel data signal 33 for downstream processing (not shown).
In the other direction, the back channel driver 34 receives the differential back channel signal 11b, and amplifies it to provide the back channel transmission signal across its termination resistance 38 for coupling to the differential signal line 30 by the coupling capacitors 40p, 40n. The back channel signal, coupled by the upstream coupling capacitors 20p, 20n, is received across a termination resistance 18 and amplified by the back channel receiver 14 to provide the back channel data signal 15.
As noted above, one technique relies on so-called “blanking transmission”, in which the back channel data is modulated into blanking intervals, e.g., vertical or horizontal blanking intervals of forward channel video data. For example, as depicted here, the back channel data pulses are inserted within the vertical blanking interval Tb as a time-domain multiplexed data signal. While this can be a reasonable solution to operate with a single cable, e.g., a common conductor pair, overall signal latency is increased, since blanking intervals are only available once every video frame. Depending upon the type of data being transmitted or the application, this can be a relatively long time interval and make a significant difference, e.g., in an accident scenario where detection and response time in microseconds are required.
Referring to FIG. 2, another technique relies upon common-mode modulation, where back channel data, e.g., control information, is conveyed via the same conductor pair using common-mode modulation. Since the forward channel data is transmitted differentially, common-mode modulation in the reverse direction will, ideally, not interfere with the forward channel data transmission. The back channel data pulses 35 are modulated at a common mode potential on both conductors of the conductor pair 30. While this is advantageous in terms of signal latency and costs (i.e., in terms of requiring fewer conductors), such technique has increased EMI due to common-mode signal spikes. This can result in otherwise problematic restrictions on the design of the forward channel receiver 32, particularly in terms of its common-mode signal rejection.
Accordingly, it would be beneficial to have a technique for providing simultaneous bi-directional communication via a common conductor pair while minimizing signal latency, EMI and the number of conductors required.